Polarization mode dispersion (hereinafter, “PMD”) is caused by variations in birefringence along the optical path that causes the orthogonal optical signal polarization modes to propagate at different velocities. The primary cause of PMD is the asymmetry of the fiber-optic strand. Fiber asymmetry may be inherent in the fiber from the manufacturing process, or it may be a result of mechanical stress on the deployed fiber. Environmental changes are dynamic and statistical in nature, and are believed to result in PMD changes that can last for variable periods of time and vary with wavelength, with the potential for prolonged degradation of data transmission.
Optical fiber exhibits PMD because of imperfections within the fiber, which induce localized birefringence. When the transmission path is long, these localized birefringent sections can combine to yield a particularly complicated polarization-dependent effect. These localized sections are known to result, for example, from eccentricities of the waveguide's core, micro-bubbles in the waveguide core and/or cladding, and strain gradients through the fiber cross-section. Mechanical stress on the fiber resulting from cabling and installation can also cause the fiber to suffer stress-induced birefringence.
In the laboratory and the field, there are reasons to artificially generate PMD in a controlled fashion.
In the laboratory, for example, a PMD emulator is desirably used to predictably and repeatably add PMD to signals generated by optical transmitters for testing optical receivers. In many cases, however, the center frequency of the optical signal being tested may not be properly aligned with the PMD spectrum generated by the emulator. Unfortunately, this misalignment reduces the predictability and repeatability of the tests being performed. Because a conventional PMD emulator cannot controllably “frequency shift” its spectrum to accommodate for the misalignment, those attempting to evaluate the PMD response of receivers and other equipment are generally forced to test undesirable and unpredictable PMD states. Often, PMD emulators include ten or more birefringent sections.
A PMD generator can also be incorporated into a specialized telecommunications sub-system called a PMD compensator. PMD compensators are used to mitigate the deleterious effects of PMD imparted on an optical data signal transmitted through an optical fiber. In contrast to PMD emulators, PMD compensators generally include only one or two birefringent sections, but such a small number of sections greatly limits the range of achievable PMD states. In order to achieve a greater operating range, it may be desirable to use PMD compensators that include more than two birefringent generator sections. Unfortunately, PMD spectra generated with more than two sections are difficult to control, subject to misalignment, and are highly frequency dependent.
It is known that polarization “mode-mixing” between adjacent birefringent sections of a PMD generator can be used to control the generated PMD state. For example, adjacent birefringent sections can be (1) optically aligned to maximize PMD, (2) optically crossed to minimize PMD, or (3) optically misaligned to generate some intermediate PMD state.
As mentioned briefly above, when the number of birefringent stages is greater than two, the generated PMD spectrum is generally frequency dependent. In contrast, when the number of birefringent stages is only one or two, the generated PMD spectrum is frequency independent. Although this independence is desirable from an alignment perspective, the limited number of stages provides a limited number of PMD states. Therefore, three or more stages are desirable to achieve a large number of PMD states, but control of devices with three or more stages can be highly unpredictable and susceptible to misalignment.
It would therefore be desirable to provide methods and apparatus to frequency shift a frequency-dependent PMD spectrum.
It would be further desirable to provide methods and apparatus for continuously frequency shifting frequency-dependent PMD spectrum without substantially changing the shape of the spectrum.